The present invention relates to an apparatus and method for monitoring the clearance between the refining surfaces of relatively rotating discs or plates, for refining fibrous cellulosic material.
In paper pulp refiners, wood chips are broken down into fibers suitable for the manufacture of paper, by the grinding action of two opposed, relatively rotating refiner discs. The refiner discs carry serrated surface refiner plates made of a white cast iron material. In traversing the clearance gap between the two refiner plates, the wood chips are mechanically reduced into fibers in the radially inner first stage breaker bar section, and the fibers are further processed by refiner plates at the radially outer region of the discs where the gap is very narrow.
The operation of the refiner requires close control of the clearance gap between the two refiner plates. If the gap is too large, inefficient refining and high expenditure of energy result. If the gap is too narrow, undesirable fiber interaction with the plate occurs, and the fibers are undesirably cut and shortened. Transients such as start up and changes in feed rate can also cause clashing of the refiner plates, thus leading to reduced plate life.
A variety of techniques have been known for the purpose of monitoring clearance gaps in process equipment. In U.S. Pat. No. 4,454,991, an AC impedance measurement method is described, wherein a high frequency signal is applied across the gap and the change in impedance associated with a change in gap is indicative of gap clearance.
In U.S. Pat. No. 4,447,011, a pressure transmitter technique is described, wherein a control signal is achieved by a counter-force produced in opposition to the force produced by the refiner drive.
In U.S. Pat. No. 4,387,339, a variable reluctance technique is described, wherein the probe in imbedded in one of the discs. Opposite polarity coils are used, in which the magnetic circuits include the gap. Opposing surfaces, which are sensed by the Hall effect, are nulled. The difference between the coil currents is then measured.
In U.S. Pat. No. 4,251,035, a capacitance technique is described, wherein the movement of gyrator crusher jaw is sensed by a capacitance measurement across hydraulic fluid between a shaft member and its support.
In U.S. Pat. No. 4,073,442, a direct current technique is described, whereby a constant voltage is applied and the resulting current across the gap is measured.
U.S. Pat. No. 3,944,146, describes an ultrasonic technique employing transducers which receive signals reflected from a reflector plate, thereby directly measuring the gap setting in a crusher.
U.S. Pat. No. 3,799,456 describes a technology in which the movement of the control shaft, by which one of the relatively rotating plates is axially adjusted, is monitored by an LVDT.
In Canadian Pat. No. 849,716, an inductance technique is described, wherein six coils are carried on one disc and a single magnet is carried on the other. The magnitude of the signal induced in the coils is a measure of local clearance distance.
As practical applications in pulp refiners, the LVDT technique of U.S. Pat. No. 3,799,456 has proven unreliable due to three significant factors. First, thermal expansion during start up causes an unknown differential length change in the disc shaft and base. Second, deflections of the refiner disc peripheral regions due to high pressures generated in the grinding process, are not necessarily manifested in the shaft movement. Finally, measurements of shaft length changes do not take into account wear of the refiner plates.
The variable reluctance technique described in U.S. Pat. No. 4,387,339 provides a direct means of measuring the gap between the plates and is, in that sense, an improvement over the LVDT technique. However, because the transducer is imbedded in a rotating member, the installation of the device and the transmission of power and electrical signals are very complicated. The device is also subjected to high accelerations and failure of its mechanical parts is highly probable. Frequent replacement of the transducer is also necessary since the transducer is subjected to the erosive and corrosive environment of the pulp slurry.